How reorder layers in animation? (grease pencil)
There are two ways to go about this, not including the one you have.
The first is separating each ball onto its own object, then translating the objects’ Y axis to literally move one ball behind the other. Grease pencil objects are flat, so a translation as small as 0.001 should suffice. This method is good if you have a lot of individual objects with their own properties because you can then parent them, like pieces of armor or orbiting planets.
The other method is the ‘switchboard’ technique, which utilizes the dope sheet. At a certain point in the animation, copy layer 1’s keyframe and paste it onto layer 2, then copy layer 2’s key frame onto layer 1 - manually switching the two. This is good if you have a lot of components to a single object, say like fingers on a hand.